The present invention relates to a pickup cartridge for reproducing disk-shaped record carriers having an information track in the form of a spiral groove on which audio and/or video information is recorded. More particularly, the present invention relates to a pickup cartridge comprising a shielded casing, a cantilever which at its one end being pivotally supported within the casing and projecting from the casing through an opening formed in a wall of the casing, a stylus carried by the cantilever at its free end, the opening during reproduction of the record carriers allowing omnidirectional movement of the stylus and the cantilever about the pivoted end of the cantilever, and a transducer arranged within the casing in the vicinity of the opening.
The pickup cartridge of this kind is known from, for example, Japanese Patent Application Laid Open Publication No. 8,502/73. In such a pickup cartridge, since the transducer is close to the stylus, the cantilever is used only as a carrier of the stylus, and not for transmitting the movement of the stylus to the transducer. Thus, audio and/or video information recorded on the record carrier and detected as mechanical vibration of the stylus is transduced directly, by means of the transducer, into electrical signal without being affected by the compliance of the cantilever. By this reason, the weight of the cantilever, and hence the effective moving mass of the pickup cartridge can be greatly reduced.
Because of the undulation of the spiral groove of the record carrier or unevenness of the surface thereof, the stylus at the free end of the cantilever is forced to move omnidirectionally about the pivoted end of the cantilever substantially in a plane perpendicular to the longitudinal axis of the cantilever by an amount of approximately 1.5 mm. The opening formed in a wall of the shielded casing of the cartridge is to allow such a movement of the stylus. This opening, however, permits entry of foreign matters, such as moisture, dust or fine particles which are liable to accumulate onto the transducer deteriorating characteristics thereof. This undesired entry of the foreign matters takes place under electrostatic or electromagnetic force induced by the transducer.
In order to avoid an accumulation of the foreign matters onto the transducer thereby maintaining the characteristics of the cartridge always optimum, the present inventor proposed in Japanese Patent Application Laid Open Publication No. 159,358/77, to dispose in the vicinity of the opening and within the casing, a dummy transducer in the form of electrette which serves to collect the foreign matters thereon. By this arrangement, the actual transducer used to transduce the information is free from the foreign matters. Nevertheless, the total performance of the pickup cartridge thus obtained was not satisfactory since, in order to obtain optimum collection efficiency of the dummy transducer, this transducer has to be disposed close to the opening where the actual transducer would have to be positioned to obtain the optimum characteristics of the pickup cartridge.